Complementary Metal Oxide Semiconductor (CMOS) devices are dominated by n-channel (NMOS) and p-channel (PMOS) transistor structures. Various physical characteristics of each type of transistor determine the threshold voltage (Vt) that must be overcome to invert the channel region and cause a given transistor to conduct majority carriers (either by electrons movement in an NMOS device or by hole movement in a PMOS device).
One of the controlling physical characteristics is the work function of the material used to form the gate electrode of the transistor device. In semiconductor devices, such as a Dynamic Random Access Memory (DRAM) device, the transistor gates are predominantly made of polysilicon and an overlying layer of tungsten silicide. However, polysilicon transistor gates are prone to polysilicon depletion effects such as unwanted voltage drops that reduce transistor performance.
Metal gate technology presents an alternate approach for CMOS transistor devices in that the traditional polysilicon gate electrodes are replaced with metal or metal alloy electrodes. In order to substitute the traditional p-type or n-type polysilicon transistor gate with a metal or metal compound it is desirable to use metals that have similar work function characteristics in order to obtain a comparable transistor threshold voltage. N-type doped silicon has a work function of approximately 4.15 electron-volts (eV). Metals having a similar work function include aluminum (Al), manganese (Mn), zirconium (Zr), niobium (Nb), hafnium (Hf), and tantalum (Ta). P-type doped silicon has a work function of approximately 5.2 eV. Metals having a similar work function include nickel (Ni), cobalt (Co), platinum (Pt) and ruthenium (Ru).
Currently, in the semiconductor fabrication, there is a need for a metal that can be used to form both the gates of both n-channel and p-channel transistors in semiconductor devices, a need of which is addressed by the following disclosure of the present invention that will become apparent to those skilled in the art.